The present invention relates to a semiconductor optical integrated device and its fabrication method, as well as to a semiconductor optical integrated device as part of an optical communication module and optical communication system. The present invention is also directed to a light receiver, particularly to a light receiver having a structure suitable for a polarized-wave diversity light receiving system used for coherent optical communication, which light receiver can include the semiconductor optical integrated device.
It is known that the characteristics of semiconductor optical functional devices such as a semiconductor laser, an optical modulator, a light switch, an optical sensor, and an optical amplifier can greatly be improved by using a superlattice structure having optically biaxial strain.
It is estimated that monolithic integration of these devices is indispensable in the future, in order to advance optical devices such as the foregoing, and in order to advance light application techniques. However, current crystal-growth techniques cannot integrate superlattice structures having different strains, e.g., on a single substrate, and no embodiment of such semiconductor optical functional device having a superlattice structure with optically biaxial strain has been realized so far. Therefore, polarization of the activating light in an integrated device has been restricted to the TE mode (polarization plane parallel to the crystal growth surface) so far.
To integrate different functional devices on the same semiconductor substrate, however, a method is proposed which controls the band gap energy on a substrate surface by using selective area growth. This type of semiconductor optical integrated device was reported at the Autumn Meeting, C-133, of The Institute of Electronic Information Communication, held on Sep. 5, 1991.
The aforementioned method makes it possible to integrate quantum well optical Waveguides with different quantum well layer thicknesses, or different quantum levels, on a substrate surface through a single crystal growth, by crystal-growing a quantum well structure 3 having quantum well layers 5 and quantum barrier layers 6, and sandwiched by optical waveguide layer 4 and cladding layer 7, on a semiconductor substrate 2 on which an insulating mask 1 is formed. See FIGS. 1A and 1B, each showing section "a" and section "b" having quantum well optical waveguides with different quantum well structure thicknesses. In this case, however, the quantum well layer thickness and mixed crystal composition of devices to be integrated are uniquely specified, by the necessary insulating mask width. Therefore, it is impossible to control the strain on the substrate surface, and it is difficult to apply a strain system, to the quantum well structure, in which further improvement of device characteristics can be expected. Thus, it is impossible to design an optimum device structure of a quantum well layer or the like for each semiconductor optical integrated device.
Such semiconductor optical integrated devices have use in coherent optical communication systems. For coherent optical communication, signal intensity is increased by making the beam emitted from a local oscillation laser, set at the receiving side, interfere with signal light. Moreover, high-density wavelength multiplexing is achieved by using sharp-interference wavelength selectivity at the receiving side. However, to make the locally-oscillated light interfere, it is necessary to equalize the polarized light of the signal light and that of the locally-oscillated light.
Polarized-wave diversity is one of the methods to solve the problem. Polarized-wave diversity divides locally-oscillated light into two polarized lights, with an equal output by a polarized-light separator such as a polarized-light beam splitter. Signal light with any polarized-light direction, transmitted through an optical fiber, is divided into two types of polarized light perpendicular to each other to make each of them interfere with divided locally-oscillated light. Finally, the polarized light of any signal light can be received, independently of the fluctuation of interference intensity at the receiving side to the polarized light, by equalizing each divided signal intensity.
An example of structure to achieve the foregoing is shown in the "Technical Research Report" of The Institute of Electronic Information Communication, Vol. 91, No. 340 (1991), p. 45. In this report, two sets of polarized-wave diversity devices are used, and four light receivers (two for each set) are used because the structure of a balance-type light receiver is included. Signal light is coupled with a locally-oscillated laser beam by an optical coupler. Then, the light emitted from the optical coupler is collimated by a microlens to apply it to the polarized-light separator and divide it into two types of polarized light perpendicular to each other. Each type of divided polarized light is converted into an electric signal by a light receiver. As described above, the existing polarized-wave diversity structure necessarily requires optical parts for dividing polarized light and at least two independent light receivers. Therefore, the optical system is complicated and it is difficult to obtain a high reliability from the structure, using different optical parts.